Model S

Charging efficiency

edited November -1 in Model S
I apologize if there is already a thread on this. I am interested in how much electricity is lost in the charging process. If you pump 85 KWH into your battery, I am sure that your electric meter on your house will show that you used more than 85 KWH. My question is how much more. All of Tesla's analysis assumes that there is no loss - i.e. perfect transfer - from the AC current in your house into the DC battery in the Model S. Does anyone know how much that loss is and are there any factors that impact that loss - e.g. temperature, current level... And is there anyway to measure the loss. It seems to me that knowing that is critical to being able to calculate cost per mile.


  • edited November -1
    I believe it's 92% but I'm sure someone can verify that!
  • edited November -1
    92% maximum efficiency per the specs page.
    From my experience by comparing kWh spent vs what Chargepoint reports as used during a charging session, it's about 80% or less.
    Not sure how much it depends on charging current (Chargepoint is 30A) and weather conditions (if the battery is hot, tesla is running a cooling pump during charging).
    It would be interesting to gather more data.
  • edited November -1
    @ Etzbach – Currently there is no way to know how much electric power you pull from the grid during a charge cycle unless you install a dedicated power meter in line with your charging outlet. This is unfortunate since as you correctly state, this information is needed to calculate your cost per mile to drive the Model S. The Roadster provides this info (electric power pulled from grid during charge cycle) so it is frustrating that Tesla did not include this same feature in the Model S. I have contacted the Ownership Experience Team to express how important I think it is to include this feature in a future software update. I encourage other owners to do the same. On another front, I semi-moderate (there are no official moderator in this forum) the Prioritized Software Enhancement List (a private thread for reservation holders/owners only) and this feature is number 11 of 84 suggested software enhancements in that thread.

    You can back out the efficiency from the data Tesla provides in the Calculator Section of the Go Electric > Charging page. These data imply 98% efficiency for 110V at 12A, 92% efficiency for 240V at 24A, and 94% efficiency for 240V at 40A/80A. I’m not sure I would put a lot of faith in those numbers. By way of comparison, using actual data collected while charging my Roadster I observed 82% for 240V at 70A and 54% for 110V at 12A. I did try to back out the charging efficiency during one charging cycle in my Model S by observing the voltage and current reported on the instrument panel every 15 minutes during the charging cycle. The voltage and current were pretty steady at 236V and 69A. The calculated efficiency from this rough method was 98%. This seems overly optimistic to me. I know that the charger in the Model S is improved over the Roadster so I expect a charging efficiency close to 90% for 240V charging in the Model S. The charging efficiency sergiyz mentions during a Chargepoint charging session seems low to me, but I am not familiar with Chargepoint chargers.
  • edited November -1
    Rod and Barbara - This is very interesting. Your comment about a dedicated meter gave me an idea. What if you tripped every breaker in the house except the ones that go to the dedicated 240 volt charger. Read the meter, charge for a couple of hours, then read the meter again. Only downside is that you would have to reset the clock on the microwave (and I wouldn't want to do it when my wife was watching Modern Family). Would this work?
  • edited November -1
    Rod and Barbara, I wonder if it is even possible for the car to tell how much electric power it pulls "from the grid". Where would the car's meter have to be located for that? At or before the outlet, as you mention.

    Even if the car would precisely measure the electric power that passes its charge port, it would still miss losses from the charging cable (which does emit substantial heat, based on other remarks in this forum).
  • edited November -1
    I have a <a href="">TED 5000</a> monitoring my electricity usage. If no-one beats me too it I'll measure the draw once I get my car in early January.
  • edited November -1

    I don't see why that wouldn't work.
  • edited November -1
    I have a TED5000 monitoring my garage breaker panel, which is 99.9% used to charge the LEAF. I see around 82% efficiency, if the car's measurements are accurate. Some of that could be lost in the charging dock and cables, but it is mostly just a relay and GFCI in the power circuit and not much electronics to run, so I think it is mostly in the rectifier and batteries in the car.

    I would guess the Model S would be around the same, though the battery chemistry is different (the LEAF is LiMn) so it might not be.
  • edited November -1
    @Etzback - that would work, except the meter may not be accurate enough to get a good measurement after only a couple hours. Plus, you are living without power the whole time -- much easier to monitor just that circuit, it isn't very expensive.
  • edited November -1
    I guess I should switch over to 110v if I just drove a few miles and/or for maintenance charge?
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